1. Field of the Invention
The present invention relates to an antenna and a radio IC (Integrated Circuit) device, and particularly to an antenna capable of transmitting/receiving electromagnetic waves with a high gain and a radio IC device including a radio IC chip used for an RFID (Radio Frequency Identification) system.
2. Description of the Related Art
Generally, it is known that electromagnetic waves travel with an electric field component and a magnetic field component being substantially perpendicular to each other and that the energies of both components are substantially equivalent to each other. A conventional dipole antenna has been used for transmitting/receiving only an electric field component of electromagnetic waves, whereas a conventional loop antenna has been used for transmitting/receiving only a magnetic field component of electromagnetic waves. That is, conventional antennas have been used for transmitting/receiving energy of one of an electric field component and a magnetic field component, and further improvement in terms of gain is not expected.
Recently, an RFID system has been developed as a system to manage articles. In the RFID system, a reader/writer that generates an induction field communicates with an IC chip (also called an IC tag or a radio IC chip) that is attached to an article or a container, for example, and that stores predetermined information in a noncontact manner so as to transmit the information. The device described in Japanese Unexamined Patent Application Publication No. 2000-311226 is a known radio IC device on which an IC chip is mounted.
In this radio IC device, a first antenna coil disposed on a rear surface of an IC chip and a second antenna coil disposed on a module substrate face each other and magnetically couple to each other, so that signals are transmitted/received between the IC chip and the second antenna coil. On the module substrate, a third antenna coil is also disposed on an outer side of the second antenna coil, and signals are transmitted/received by the third antenna coil to/from a reader/writer.
However, in order to achieve coupling between the first antenna coil disposed on the rear surface of the IC chip and the second antenna coil disposed on the module substrate, the distance between the first and second antenna coils must be controlled to be about 20 μm or less, and the IC chip must be precisely mounted on the module substrate such that the first and second antenna coils precisely overlap each other in a plan view. A desired characteristic may not be obtained using ordinary facilities. In order to precisely mount IC chips at small intervals, high-performance apparatuses to manufacture thin films for antenna coils and to mount IC chips are required, which significantly increases the manufacturing costs.
Furthermore, an operating frequency of the third antenna coil to transmit/receive signals to/from the reader/writer depends on the length of the third antenna coil, and thus, the size of device increases when a low frequency is used.